The present invention relates to a method for manufacturing a sensing element, in particular for determining the oxygen content in exhaust gases of internal combustion engines.
Sensing elements are configured, for example, as so-called planar sensing elements, which comprise a composite construction of individual layers arranged one above another in film form. The individual films of this composite construction are arranged one above another in defined fashion, resulting in various functional layers. The individual films of the composite construction are, for example, laid onto one another by means of a screen printing technique, as green films. The sensing elements usually have solid electrolyte films, electrode films, heating, conductor films, insulation films, and protective films. Conventional sensing films may also have substrate films with printed electrolyte layers, aluminum oxide substrate films with semiconductor sensors (TiO2, SrTiO3). Instead of laying the green films onto one another, the latter, may also be obtained by individual printing steps. The arrangement of these different films one above another results in a laminated composite construction from which the sensing element is obtained by sintering.
During testing of the sintered sensing element, or during utilization thereof as intended, the individual layers of the sensing element are exposed to different temperatures. Because of these sudden temperature changes which occur with differing intensity, the sensing elements experience a temperature shock which leads to the occurrence of mechanical stresses in the surface region, in particular at the edges of the sensing element. In order to increase the temperature shock resistance of the sensing elements, U.S. Pat. No. 5,144,249 describes blunting the edges of the sensing element, i.e. equipping them with a chamfer. Chamfering is accomplished by way of a grinding operation sintering and after sectioning of the sensing elements. It is disadvantageous to subject completed sensing elements to a mechanical machining operation which is relatively complex and may lead to undesired damage to the sensing elements.
The method according to the present invention offers, in contrast, the advantage that blunting of the edges of the sensing element may be accomplished in a simple manner without the risk of impairing the sensing element. The edges of the sensing element are blunted prior to sintering, as a result, is possible to blunt the edge in any desired geometry using simple, non-chip-removing methods. In particular, blunting of the edges may be accomplished in a form deviating from a flat surface, for example in a convex or concave form, so that mechanical stresses which occur as a consequence of a temperature shock to the blunted edges cannot result in the creation of cracks.
In a preferred embodiment of the present invention, provision is made for the edges to be blunted by shaping, preferably by stamping of the film composite construction present in the green state. It is thereby possible, using a simple stamping tool, to shape the edges of the composite construction of green films in simple fashion, due to their soft consistency prior to sintering. By configuring a corresponding stamping tool, a blunting of the edges may be executed in any desired form. It is particularly advantageous if, once the stamping tools have been used, the stamping films laid in place are ones which allow shaping only of the edge region of the sensing element, and leave the other regions, in particular the planar regions of the sensing element, unmodified. In order to prevent adhesion of the green film composite construction of the sensing element in the stamping tool, the stamping film may be advantageously equipped with an anti-adhesion coating, in particular Teflon.
In a preferred embodiment, blunting of the edges is accomplished by use of a laser treatment. Use of the laser treatment makes it possible to advantageously accomplish noncontact blunting of the edges of the sensing element in the green state, so that any mechanical loads on the green film composite construction may be ruled out. It is possible to advantageously adjust the contour of the blunted edges of the sensing element by using a mask of an excimer laser.
By using the laser treatment, blunting of the edges may preferably be accomplished even before sectioning of the green films present in the composite construction, so that blunting of the edges may be accomplished very effectively. At the same time, the break points of the wafer, with the individual sensing elements may thereby be defined.
It is also preferred, in particular, if the laser treatment simultaneously accomplishes blunting of the edges and sectioning of the green film composite construction. By adjusting the laser output and the geometry of the laser beam, it is thus possible to accomplish edge blunting and sectioning in one operation.